Testing device



1932- Y A. M. CURTIS 1,869,884

TESTING DEVICE Filed July 9, 1930 BPF INVENTOP A. M Cupr/s A7 O/P/VE)Patented Aug. 2, 1932 UNITED STATES PATENT OFFICE AUSTEN M. CURTIS, EASTORANGE, NEW JERSEY, ASSIGNOR TO BELL TELEPHONE LABORATORIES,INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK TESTINGDEVICE Application filed July 9,

This invention relates to testing devices, and more particularly to adevice and method for testing the stress in a member under tension.

In many instances it is desirable to adjust the stress in a member undertension and unless there is some guidance as to the amount of tension inthe member during this adjustment the member may very easily be- 19 comeoverstressed. As'an example, in string galvanometers and in light valvesthe tension in the strings must frequently be ad-' justed in order tocompensate for changes in the temperature, etc.

In providing a device for determining the amount of stress in a memberunder tension, advantage is taken of the fact that a metallic stringwhen plucked and subsequently allowed to vibrate in a magnetic fieldgener- 29 ates an alternating E. M. F. the frequency of which is-thenatural frequency of the strings vibration. By utilizing thisalternating E. M. F. it is possible to give an audible signal in a pairof telephone receivers 25 Without at the same time damping the string soseverely that the musical character of the note is lost.

' In accordance with the preferred form of this invention the string tobe tested is vi- 3 brated in a magnetic field and the alternating E. M.F. thus produced is translated by any suitable means into a musicalnote. The tension in the string is then adjusted until the musical noteproduced corresponds to a standard musical note.

In accordance with another form of this invention the E. M. F. producedby the vibrating string is applied to the input of ,a narrow band passfilter, the output of which contains a pair of telephone receivers. Inthis embodiment of the invention a musical note will be, produced in thetelephone receivers only' when the string is adjustedto vibrate at afrequency within the transmission band of the filter.

A clearer understanding of this invention may be had by reference to theaccompanying drawing wherein Figs. 1 and 2 represent various embodimentsof this invention.

Referring to Fig. l a metallic string 1980. Serial No. 466,732.

such as the string ordinarily used in string galvanometers, or in lightvalves is mounted between the poles N and S of a permanent magnet thusmaintaining the string in a constant magnetic field. One endof string 10is secured to a stationary support 11 and the other end of the string isfastened to lever arm 12, one end of which is pivoted at 13 and theother end of which terminates under the end of adjustable screw 14 whichscrew is in threaded engagement with the extending portion 15 of astationary supporting member 16. The tension of the string 10 may bevaried by simply raising or lowering the screw 14. A relay 17 carryingarmatures 18 and 19 is arranged to vibrate at a comparatively lowfrequency by power supplied from battery 20 through an ordinary bellringin circuit the rate of vibration being control ed by condenser 21which is shunted around the winding of relay 17. Armature 19 invibrating opens and closes a circuit from battery 22, through resistance23, string 10, lever arm 12, armature 19, contact 24, back to battery.The current in passing through string 10 causes it to be attracted andrepelled by the stationary poles N and S, the rate of attraction andrepulsion being controlled by the natural period of vibration ofarmature 19. This causes the string 10 to vibrate in the steady magneticfield which exists between these poles, the vibration being eflectivelydamped by resistance 23. This vibration of strin 10 in a magnetic fieldsets up an alternatlng E. M. F. which is translated into audible signalsby means of an A. C. detecting circuit. This detecting circuitcomprisesa variable condenser 25 and an inductance coil 26 shunted around theterminals of the string, the inductance coil 26 being in turn shunted bya pair of telephone receivers 27. This circuit is chosen so as to berelatively high in impedance for all frequencies, but is tuned-approximatel to the frequency to which it is desired to ad ust thestring, the tuning being accomplished by 95 properly proportioning therelation between the condenser 25 and the inductance coil 26.

In operation the procedure is simply to start the vibration of armature19 by closing switch 28. This opens and closes the circuit 10cthroughthe string under adjustment and causes it to vibrate in thesteady magnetic field which exists between the poles N and S.

This vibration of the string in the magnetic ly diiferentarrangement foraccomplishing.

the same result. In this arrangement a string 30 is mounted between thepoles N and S of a permanent magnet in a manner similar to that inwhich. string 10 is mounted in the previous embodiment of thisinvention. .An armature 31 is arranged to vibrate in a manner similar tothe manner in which armature 19 of relay 17 is vibrated and thisarmature in vibrating opens and closes a circuit from battery 32,through resistance .33, primary winding 34, armature 31 contact 35, backto battery. This generates short impulses in the secondary winding 36and string 30 is vibrated accordingly in the magnetic field. whichexists between poles N and S. The note produced by the vibration of thespring is also detected by an A. C. detecting circuit. This circuitcomprises a pair of telephone receivers 37 connected to the output of anarrow band pass filter 38' which is connected across the terminals ofspring 30. The input circuit of the narrow band pass filter and thesecondary circuit of the transformer are protected by resistances 33 and39 respectively, which resistances are sufliciently high to avoidappreciable electrical damping of the string. In this embodiment of theinvention a musical note is heard in the telephone receivers only whenthe string is adjusted to vibrate at afrequency within the transmissionband of the filter 38.

In operation the procedure isto adjust the band pass filter and to startarmature 31 vi-;

brating. The tension on string 30 is then ad-' justed until amusicalnote is heard in the tele hone receivers.

rom the foregoing it is obvious that when greater accuracy of adjustmentis required the-method oftestmight be extended by con necting a vacuumtube amplifier between the 4 string terminals and the A. C. detectingcirment to be made as great 'as might be desired:

F., translating the transmitted frequencies into a musical note andcomparing said note with a musical note of a given frequency.

2. The method of adjusting a member under tension so that the stress insaid member is .within predetermined limits which consists in vibratingsaid member in a constant magnetic field to generate an alternating E.

M. F., transmitting a narrow band of fre-- quencies while suppressingall other frequencies of said alternating E. M. F., translating thetransmitted frequencies into a musical note and varying the .tension ofsaid vibrating member until its natural period of vibration issubstantially equal to the transmitted frequency of the alternating E.M. F.

3. A device for testingthe stress in a member undertension comprisingmeans for producing a steady magnetic field, means for vibrating saidmember in said field to generate an alternating E. M. F. and a tuneddetecting circuit to measure the frequency of said generated alternatingE. M'. F.

4. A device for testing the stress in a number under tension comprisingmeans for vibrating said member in a constant magnetic field to producean alternating E. M. F.,

means including a tuned detecting circuit for translating saidalternating E. M. F. into a musical note, and means for varying thetensionof said member to bring the frequency of said note withinpredetermined limits.

5. A device for testing the stress in a member undertension comprisingmeans for pro ducing a steady magnetic field, means for vibrating saidmember in said field to generate an alternating E. M. F., an A. C.detecting circuit to measure the frequency of said-generated alternatingE. M. F., and means included in said detecting circuit to tune saidcircuit to a desired frequency.

I 6. In a device for adjusting the stress in a member under tension, apermanent magnet for producing a steady-magnetic field, a vibratorycircuit comprising a source of power and a relay for vibrating saidmember in said field to generate an alternating E. M. F., means foradjusting the tension in said memher to vary the frequency .of saidalternating E. M. F., means included in said circuit for damping thevibration of said member, an alternating current detecting circuit tomeasure the frequency of said generatedalternating E. M. F., and meansincluded in said detecting circuit to tune said circuit to adesiredfrequency.

7. In a device for adjusting the stress in a member under tension, apermanent magnet for producinga steady magnetic field, means formounting said member in said field, a vibratory circuit comprising asource of power, and a double contact relay for vibrating said member insaid field to generate an alternating E. M. F., means for adjusting thetension in said member to vary the he quency of said alternating E. M.F. means included in said current for damping the vibration of saidmember and an alternating current. detecting circuit to measure thefrequency of said alternating E.- M. F., said circuit comprising avariable resistance and an inductance coil shunted across said memberand a pair of telephone receivers shunted across said inductance coil.

In witness whereof, I hereunto subscribe my name this 8th day of July,1930.

AUSTEN M. CURTIS.

